1. Field of the Invention
The present invention relates generally to a measurement processing arrangement for measuring high frequency periodic measured signal curves.
2. Description of the Related Art
Sampling methods or a combination of sampling and averaging methods is often used for measuring signal curves of periodic, high-frequency signals. The sampling method provides a high time resolution since chronologically offset segments are taken from a series of periods and combined in a slower, new signal having the same shape. The averaging method, on the other hand, provides good signal-to-noise ratios since the measured value is multiply sampled over a series of periods and then averaged. Both the sampling method and the averaging method are used in oscilloscope technology, as well as in electron beam testing. In electron beam testing technology, electrical voltages at the surface of an integrated circuit are imaged and measured. Most integrated circuits must be dynamically tested, which means that they are operated at nominal frequencies during the testing investigation.
The starting point of the development of electron beam testing is documented in "Voltage Contrast" from Phys. B. 38 (1982), No. 8, Elektronenstrahl-Messtechnik zur Pruefung integrierter Schaltungen, by Hans Rehme. In a scanning electron microscope, a primary electron beam is scanned across the surface of a specimen to be imaged through the use of deflection coils. A detector senses secondary electrons triggered by the primary electron beam, and measurement processing arrangements connected at the output of the detector use the measured signals to modulate the intensity of a picture screen.
Circuit arrangements, known as boxcar integrators, are used in electron beam testing for the combination of sampling and averaging methods. The use of boxcar integrators is shown in U.S. Pat. No. 4,486,660, entitled "Electron Beam Testing Device for Stroboscopic Measurement of High-Frequency Periodic Events". The boxcar integrator is essentially composed of a phase control unit, a delay unit, a gate circuit, and a measurement processing unit.
A block diagram of a boxcar integrator, as well as its interconnection in an electron beam testing apparatus, is shown in FIG. 10 on page 20 of the periodical Scanning, Vol. 5 14/24 (1983) of the article entitled "Electron Beam Testing: Methods and Applications", by H.P. Feuerbaum. An example of a boxcar integrator which works in a combined sampling and averaging method is Model 162 from Princeton Applied Research (see Model 162 Boxcar Averager EG & G, Princeton Applied Research Operating and Service Manual).
In German Published Application No. DE 31 38 992 Al, is disclosed a sampling method for fast voltage measurement in electron beam testing technology. The fast voltage measurement is achieved by multiply sampling the signal curve by a pulsed electron beam during a period of the periodic signal curve. However, the German Application No. DE 31 38 992 Al does not disclose any particulars as to how the sampling method is to be implemented. Moreover, it is limited exclusively to electron beam testing technology.
A disadvantage in the use of boxcar integrators as hitherto employed is that the high measuring time is derived from the maximum measuring rate of the boxcar and from the sampling method. In the sampling method, n sampling points are measured in n periods of the measured signal. Spacing between the zero crossing axis of the measured signal and the sampling point changes with an increasing number of sampling points. In the sampling and averaging method, the time interval over which the signal is to be measured is divided into n generally equidistant sampling points and the measured signal is sampled m times for every sampling point. This operation is repeated for the next sampling point, so that the measuring time amounts ##EQU1## (where T being equal to the period of the measured signal). In measurements of extremely fast signals using extremely short electron pulses, a greater number of measurements must be carried out, which leads to measuring times spanning a number of minutes. Due to the instability of the measuring equipment and to the phenomena of drift, such long measuring times lead to extremely high measuring errors.